1. Field of the Invention
The present invention relates to coding and decoding audio data, and more particularly, to a method and apparatus for coding audio data so that a coded audio bitstream has a scalable bitrate, and a method and apparatus for decoding the audio data.
2. Description of the Related Art
Due to recent developments in digital signal processing technology, audio signals are generally stored in most cases as digital data and reproduced. Digital audio storage/restoration apparatuses transform audio signals into pulse code modulation (PCM) audio data, i.e., digital signals, through sampling and quantization. By doing so, the digital audio storage/reproducing apparatus stores the PCM audio data in an information storage medium such as a compact disc (CD) and a digital versatile disc (DVD), and reproduces the stored signal in response to a user's command such that the user can listen to the audio data. The digital storage/restoration method greatly improves audio quality compared to analog methods using a long-playing (LP) record or magnetic tape, and dramatically reduces deterioration caused by a long storage period. However, the digital method has a problem in storage and transmission due to the large amount of digital data.
To solve this problem, a variety of compression methods are used to compress digital audio signals.
In Moving Pictures Expert Group (MPEG)/audio standardized by International Standard Organization (ISO), or AC-2/AC-3 developed by Dolby, the amount of data is reduced using psychoacoustic models. As a result, the amount of data can be efficiently reduced regardless of the characteristics of a signal. That is, the MPEG/audio standard or AC-2/AC-3 method can provide almost the same audio quality as that of a CD with a bitrate of only 64˜384 Kbps, which is ⅙ to ⅛ of that of the previous digital encoding method.
In these methods, however, an optimal state suitable for a fixed bitrate is searched for and then quantization and encoding are performed. Accordingly, if the transmission bandwidth is lowered due to poor network conditions in transmitting bitstreams through the network, cut-offs may occur and appropriate services cannot be rendered to a user any more. In addition, when the bitstream is desired to be transformed into bitstreams of a smaller size more suitable for a mobile apparatus having a limited storage capacity, a re-encoding process should be performed in order to reduce the size of a bitstream, and the amount of computation required increases.
To solve this problem, the applicant of the present invention filed Korea Patent Application No. 97-61298 on Nov. 19, 1997 entitled “Bitrate Scalable Audio Encoding/Decoding Method and Apparatus Using Bit-Sliced Arithmetic Coding (BSAC)”, for which a patent was granted on Apr. 17, 2000 with Korea Patent No. 261253. According to the BSAC technique, a bitstream coded with a high bitrate can be made into a bitstream with a low bitrate, and restoration is possible with only part of the bitstream. Accordingly, when the network is overloaded, or the performance of a decoder is poor, or a user requests a low bitrate, services with some degree of audio quality can be provided to the user by using only part of the bitstream, though the quality will inevitably decrease in proportion to the decrease in the bitrate.
However, since the BSAC technique adopts arithmetic coding, complexity is high, and when the BSAC technique is implemented in an actual apparatus, the cost increases. In addition, since the BSAC technique uses a modified discrete cosine transform (MDCT) for transformation of an audio signal, audio quality in a lower layer may severely deteriorate.